Breathing frequencies of northern elephant seals at sea and on land revealed by heart rate spectral analysis

Elephant seals breathe episodically at sea and on land and surprisingly lon g apnoeas occur in both situations. An important difference is that recover y from apnoeic periods is much quicker at sea, which might be due, in part, to differences in the ventilatory response. Respiratory frequencies of juv enile northern elephant seals diving at sea and resting on land were estima ted from time-frequency maps of the Wigner distribution of heart rate varia bility. Simultaneous direct measurement of respiration and estimation of re spiratory frequency (fR) in the laboratory demonstrated that the error of e stimation was small (mean +/- S.D. = 1.05 +/- 1.23%) and was independent of the magnitude of fR. Eupnoeic fR at sea was 2.4 times higher than on land (22.0 +/- 2.0 vs. 9.2 +/- 1.3 breaths min(-1) respectively), facilitating q uick recovery from the preceding dive and allowing a 34% increase in time s pent apnoeic at sea versus on land. The overall fR (no. of breaths in a eup noea divided by the total time of the apnoea + eupnoea cycle) of 2.3 +/- 0. 6 breaths min(-1) at sea was no different from the rate on land and was inv ersely related to the preceding dive duration, suggesting that metabolism o n longer dives may be reduced. (C) 2000 Elsevier Science B.V. All rights re served.